The plan of the Kings of Tor could work, with some qualifications.
INHERITANCE IS A ONE WAY STREET
The plan of the Kings of Tor to acquire land by marrying off their sons and daughters to other noble houses wouldn't work. Or rather, the marrying off of the daughters to members of other royal or noble houses wouldn't work for the Kings of Tor.
For example, King William the Evil Usurper who conquered England in 1066 was related to the former King Edward the Confessor who died in 1066. William's grand aunt Emma was the wife of King Aethelred the Unready of England and the mother of King Edward the Confessor.
So, because King Edward the Confessor of England was descended from earlier Dukes of Normandy through his mother, King Edward the Confessor had a legal claim to the Duchy of Normandy. If Duke William of Normandy and all his children and close relatives died, King William the Confessor of England could have claimed Normandy and might have acquired it.
But, despite what some illogical people say, Duke William of Normandy had no claim to the Kingdom of England through that relationship. Duke William of Normandy was not descended from any previous King of England. He wasn't even descended from any English serfs or slaves.
When the daughter of a king or lord marries someone, she transmits a potential to inherit her father's lands or part of them to her husband and her descendants. But she doesn't transmit any potential claim to inherit her husband's lands from her husband to her father and brothers.
So if A, King of Tor, married his daughter B to Lord C of the fief D, and they had a son Lord E of the fief D, Lord E of the fief D would have a potential claim to inherit the Kingdom of Tor. If everyone ahead of him died, and if the inheritance rules of Tor permitted inheritance through females, Lord E of the fief D could inherit the entire Kingdom of Tor, or a part of it.
If F, King of Tor, married his son and heir G to lady H of the fief I, and they had a son and heir J, J would be in line to inherit the Kingdom of Tor from his father G and grandfather F. And if Lady H's brothers all died without children, and if the laws of fief I allowed inheritance through females, J could inherit part or all of fief I.
And in the European Middle Ages most lords, barons, viscounts, counts, landgraves, margraves, counts palatine, princes, dukes, kings, emperors, etc., considered the possibilities of inheritance when choosing who to marry their children to. They would guess which eligible females were mostly likely to become heiresses when choosing brides for their sons, and when choosing husbands for their daughters they would guess which families they would least mind having their lands inherited by in the future.
THE MARRIAGE CHOICES OF EMPEROR MAXIMILIAN I (1459-1519)
The workings of fate, or blind chance, or something, really favored the matrimonial choices of Maximilian I, Elected Emperor of the Romans and hereditary Archduke of Austria, Duke of Styria, etc.
First he married in 1477 Mary, Duchess of Burgundy, etc., etc. and after her death was often the regent for their son and heir Philip the Handsome.
Then he negotiated two marriage alliances with King Ferdinand II of Aragon, etc. and his wife Queen Isabella I of Castile, etc.
Maximilian's son and heir Philip the Handsome married Joanna La Loca, daughter of Ferdinand & Isabella, in 1496. Ferdinand and Isabella's son and heir John, Prince of Asturias, married Maximilian's daughter Margaret in 1797.
If everything went well for everybody, Philip, the ruler of Burgundy, etc., would have inherited Austria, etc, from his father, and John would have inherited Aragon, etc., and Castile, etc. from his parents.
But John, Prince of Austurias, died childless in 1497, and his widow gave birth to a stillborn child. John's oldest sister Isabella, Queen of King Manuel I of Portugal died in 1498, and her only child Miguel de Paz, eventual heir of Portugal, Aragon, etc., and Castile, etc., died in 1500.
That left John's next sister Joanna la Loca as the heiress of Ferdinand and Isabella. When Isabella died in 1504. Joanna became the Queen of Castile, etc., and Philip the Handsome became King of Castile, etc., as her husband. But Philip died in 1506, and King Ferdinand II took over ruling Castile, etc., in the name of his daughter Joanna.
Philip the Handsome and joanna's oldest son and heir Charles, eventually inherited all the lands of his four grandparents.
Charles inherited Austria, etc. from his paternal grandfather Maximilian I in 1519, Burgundy, etc., (which came from his paternal grandmother Mary) from his father Philip in 1506, Aragon, etc., from his maternal grandfather in 1516, and castile, etc. from his maternal grandmother Isabella in 1516. Technically Charles didn't inherit Castile, etc., from his mother Joanna la Loca until she died in 1555, but he became co King of Castile, etc. with his mother Joanna in 1516 and ruled in her name.
In 1515 Maximilian met with Sigismund I, King of Poland and Grand Duke of Lithuania, and his brother Vladislaus II, King of Hungary, etc., and King of Bohemia, and negotiated a mutual succession treaty and arranged another double marriage, between Vladisaus's son and heir Louis and Maximilian's granddaughter Mary, and between Maximilian's younger grandson Ferdinand and Vladisaus' daughter Anna.
Ferdinand married Anna in 1521 and Louis married Mary in 1515. King Louis was killed in the Battle of Mohacs against the Turks in 1526 without any legitimate Children, and Ferdinand became King of Bohemia and King of part of Hungary.
So by fate or chance or something Emperor Maximilian I's matrimonial decisions paid off really, really, really big time, thus inspiring the famous rhyme:
Bella gerant aliī, tū fēlix Austria nūbe/ Nam quae Mars aliīs, dat tibi regna Venus, "Let others wage war, but thou, O happy Austria, marry; for those kingdoms which Mars gives to others, Venus gives to thee."
And basically about 99 percent of the noble and royal marriages in western Europe for many centuries before and after the time of Maximilian I were arranged with at least a little thought and hope that something like this might happen to benefit one family or the other.
ACQUIRE FIEFS IN OTHER KINGDOMS BY MARRIAGE - YES
So it would be perfectly possible for the Kings of Tor to acquire small fiefs in other kingdoms by married women whose descendants would eventually inherit those fiefs.
In 1120, William, only legitimate son of Henry I, King of England and Duke of Normandy (son of William the Evil Usurper mentioned above), died in the White Ship disaster. Henry's only legitimate daughter, Empress Matilda (1102-1167), was married to Emperor Henry V. Emperor Henry V died childless in 1125 and Matilda returned to England and Normandy.
Henry I married Matilda to Geoffrey "Plantagenet" (1113-1151) in 1128. Geoffrey's father Fulk named him Count of Anjou and Maine in France in 1129 and went to become King of Jersusalem. King Henry I died in 1135, and civil war erupted in England and Normandy between Empress Matilda and Stephen Count of Blois.
Eleanor of Aquitaine (1122/24-1204) inherited the large and wealthy Duchy of Aquitaine, etc. in France in 1137, making her the most desirable heiress in Europe. Her overlord and thus guardian, King Louis VI of France, whose own royal domain was much smaller than Aquitaine, married her - surprise, surprise - to his son and heir King Louis VII of France in 1137.
In 1152 King Louis VII had his marriage with Eleanor, which had produced only girls, annulled. Eleanor promptly married another French vassal, Henry (1133-1189) Duke of Normandy, Count of Anjou and Maine, etc., the oldest son of Empress Matilda and already an over mighty vassal of the King. Now Henry and Eleanor ruled about as much land as all the other French lords - including the King - put together, and were vastly more powerful than the King.
Then, in 1154, King Stephen of England, Matilda's rival, died and Henry became King Henry I of England. And for three centuries the history of England and France was was dominated by the fact that the King of one country, England, owned various fiefs that totaled about half the area of France. The Kings of England were totally independent of France in England, but in France they were legally merely dukes and counts of various fiefs and thus vassals of of the King of France.
Since in this extreme historic case the Kings of France were unable to prevent foreign Kings from inheriting huge fiefs in France, half the entire Kingdom, you can expect that in most feudal kingdoms the Kings would be unable to prevent foreign kings of inheriting various small fiefs.
So it would be perfectly possible for Kings of Tor to inherit various disconnected fiefs in one or more kingdoms along the River Goro. Keeping everyone of them forever would be a different matter and would depend on various events in the history of those fiefs and there relations with other fiefs and with various overlords who might confiscate those fiefs if angry at the king of Tor. In short, how long those fiefs were retained would depend on the fictional history in your story.
Note that the typical relationship between the Kingdom of Tor and various fiefs obtained by the kings of Tor in one or more kingdoms along the River Goro would that of a personal union. In such a personal Union the fiefs would not have their governments united with each other or with the Kingdom of Tor. The only thing the various lands would have in common was that a single person would be King of Tor and also the lord, count, duke, etc. of the various fiefs.
ACQUIRE KINGDOMS BY MARRIAGE - YES
It was obviously possible for someone to acquire other kingdoms by marriage and pass them on to their descendants.
The examples given in the section about the marriage arrangements of Emperor Maximilian I prove that, for example.
Note that King Ferdinand II of Aragon ruled several kingdoms that were separated by the sea from Aragon, Valencia, and Barcelona, including the Kingdom of Sicily, the other Kingdom of Sicily, and the Kingdom of Sardinia.
The Castilian group of Kingdoms included "The Kingdom of the Indias, the Islands and Mainland of the Ocean Sea" far across the Atlantic Ocean.
For a couple of centuries the Kings of the Spanish kingdoms also ruled the Burgundian lands, a group of fiefs in the Kingdom of Germany in the Holy Roman Empire, and some in the Kingdom of France, and all separated from Spain by hundreds of miles by land or by sea.
So it was certainly possible to acquire several separate kingdoms and fiefs that were physically sparated by water or bylands of other kingdoms and fiefs.
I note that in medieval Ireland there were allegedly about 90 or 150 kingdoms, according to different sources. The Island of Ireland has an area of about 84,421 square kilometers or 32,595 square miles.
If one assumes that there were about 50 to 200 kingdoms at any one time in medieval Ireland, the average size kingdom would have an area of about 422.105 to 1,688.42 square kilometers or 162.975 to 651.9 square miles. So if the average kingdom was a perfect square it would be about 20.545 to 41.09 kilometers, or 12.766 to 25.532 miles on each side. Someone could walk from one Kingdom across another whole kingdom and into a third during a single day.
At times when the population of medieval Ireland was amount 1,000,000 persons, the average Irish kingdom might have a population of about 5,000 to 20,000.
So if the Goro River was a very long river there could have been hundreds of such tiny kingdoms if it was several thousand kilometers or miles long, especially if those kingdoms had narrow water fronts and stretched a long way back inland from the River.
So if the Kings of Tor married princess from nearby and sometimes distant kingdoms along the Goro River, they might from time to time inherit various scattered kingdoms along the river. Some of those kingdoms might be separated from other kingdoms ruled by the King of Tor by only a few in between kingdoms, while others might be separated by tens of kingdoms.
Each and every single kingdom inherited by the Kings of Tor would normally remain a separate kingdom, independent from the Kingdom of Tor and from the other kingdoms along the river. That is called a personal union, when the only thing that two different kingdoms have in common is that the same person is the monarch of both of them. And in history that was the normal relationship between two different kingdoms that had the same monarch. It would usually take generations or centuries of having the same ruler for the two kingdoms to agree to unite into one larger kingdom.
ACQUIRE FIEFS IN OTHER KINGDOMS AND MAKE THEM PART OF YOUR OWN KINGDOM - NO, NO, NO!
The main reason why the Kings of Tor would not make fiefs they inherited in other kingdoms part of the Kingdom of Tor is that would be obviously a crime. As lords of fiefs in another kingdom, they would be vassals of another king, the king of that kingdom, and so would be legally obligated to be loyal subjects. Stealing land from his kingdom would not be loyal to him. It would be an act of treason. Treason, if unsuccessful, was often punished by death and almost always by confiscation of the fief.
And taking land out of one kingdom and making it part of another kingdom would be an obvious act of aggression by the kingdom that acquired it. Medieval rulers rarely committed (obvious) acts of aggression, which after all, set bad examples for their neighbors in the future. Medieval rulers preferred to disguise their aggression with all sorts of silly excuses.
So the Kings of Tor would probably not try to make any fiefs they inherited in other kingdoms parts of the Kingdom of Tor, not while their society was anything like medieval Euorpean society.